The SIR provided Te-Methionine; Ig One of the fundamental problems in macromol. crystallography is the availability of the suitable heavy-atom derivatives necessary to solve the phase problem. The ability to label a protein with a tellurium-contg. amino acid (telluromethionine) at internal sites through the utilization of protein biosynthesis supplies x-ray crystallographers a convenient phasing vehicle and NMR (NMR) spectroscopists an internal probe with which to study structure/function relationships via Te-125 NMR spectroscopy. In this communication we demonstrate the partial incorporation of telluromethionine into E. coli dihydrofolate reductase (DBFR) with no apparent perturbations to activity or substrate binding. Enzyme contains two moles TeMet exhibited a specific activity of 42 units/mg and a 1: 1 binding ratio with methotrexate.